It has been hitherto known in the rubber industry that when a rubber composition containing a filler such as carbon black is produced, a rubber wet master batch is used to improve the processability of the composition, or the dispersibility of the filler. This is a technique of mixing the filler and a dispersing solvent with each other at a predetermined ratio in advance, dispersing the filler in the dispersing solvent by mechanical force, mixing the resultant filler-containing slurry solution with a rubber latex solution in a liquid phase, adding a coagulator such as an acid thereto to prepare a coagulation, recovering the coagulation, and then drying the recovered coagulation. The use of the rubber wet master batch gives a rubber composition more excellent in the dispersibility of the filler therein and more excellent in rubber physical properties such as processability and reinforceability than the use of a rubber dry master batch obtained by mixing a filler with a rubber in a solid phase. The use of such a rubber composition as a raw material makes it possible to produce a rubber product such as a pneumatic tire decreased in rolling resistance and excellent in fatigue resistance.
In the above-mentioned rubber wet master batch producing method, a method for removing water originating from the dispersing solvent and the rubber latex solution, from the filler-containing rubber coagulation obtained after the coagulation step is, for example, a method of performing solid-liquid separation by filtration or centrifugation, and then using any mixer to knead the filler-containing rubber coagulation while heating, thereby performing dehydration. In this dehydrating method, the water content in the rubber wet master batch obtained after dehydration can be made lower as the number of steps including dehydrating, drying and plasticizing steps is made larger, or the heating temperature in kneading is made higher. However, as the number of the steps including the dehydrating, drying and plasticizing steps, or the heat quantity and/or mechanical energy applied in dehydrating is increased, the resultant rubber wet master batch undergoes cleavage of polymer molecular chains thereof, or the like, so that the rubber composition to be finally obtained may be deteriorated in vulcanized rubber characteristics.
Patent Document 1 listed below describes a method for producing a rubber-chemical-containing master batch, characterized by using a biaxial extruder as a first kneader to dry and knead a rubber wet master batch containing a rubber component and a filler, and then using a Banbury mixer as a second kneader to mix a rubber chemical with the master batch. However, in this producing method, mechanical energy given to the rubber wet master batch is large since this rubber wet master batch is produced by drying and kneading the filler-containing rubber coagulation, using the biaxial extruder. Thus, the rubber component therein tends to be degraded. Accordingly, the vulcanized rubber to be finally obtained tends to be deteriorated in tearing resistance, and stress characteristics at a high strain region of the rubber.
Patent Document 2 listed below describes a method for producing an elastomer composite material, including performing the step of masticating a rubber coagulation obtained after a dehydrating step while a mechanical energy of at least about 0.3 MJ/kg (about 249 W/kg) is given to the coagulation; and subsequently performing an additional masticating step of masticating the coagulation while a mechanical energy of about 0.9 MJ/kg (about 83 W/kg) is given to the coagulation. However, also in this producing method, the mechanical energy given to the rubber wet master batch is large so that the rubber component therein tends to be degraded. Furthermore, the dehydrating and drying steps are performed several times to intensify the tendency that the degradation of the rubber component is caused. Consequently, the vulcanized rubber to be finally obtained tends to be deteriorated in tearing resistance and stress characteristics at the high strain region.